The present invention relates to apparatus and methods for detecting radon contamination, and is particularly concerned with a radon detection system incorporating environmental monitoring and tamper detecting capabilities to enhance the accuracy and reliability of radon level measurements.
In recent years, the infiltration of radon gas into homes and other structures has been recognized as a significant health risk. Radon 222 is a colorless, odorless gas which is produced by the natural decay of underground uranium deposits. The decay products of radon gas, principally radon A (polonium 218) and radon C' (polonium 214), attach to airborne dust particles which in turn can become lodged in the lungs. It is believed that prolonged exposure to these particles can lead to lung cancer.
Although measurable levels of radon gas can be found virtually everywhere, indoor environments are of particular concern. If a significant amount of radon gas is present at the site and is able to penetrate into the structure, indoor radon concentrations can reach dangerously high levels. Under present guidelines, an average radon gas concentration in excess of 4 picoCuries per liter of air is regarded as sufficient to require that corrective action be taken. Usually, such action involves providing additional ventilation or taking steps to prevent the entry of radon gas into the structure.
A number of continuous monitoring systems have been developed for measuring radon levels in underground mines and other environments in which high levels of radon contamination have been observed. However, these systems are not cost-effective for widespread radon testing in homes and other structures, since they are often quite complex and must be operated by trained personnel to insure proper set-up and calibration. For this reason, home radon testing has generally been carried out using a simple passive device consisting of a canister of activated charcoal which is exposed to the ambient air inside the structure to be tested. The canister is left in place for a predetermined amount of time, during which the charcoal adsorbs radon gas from the surrounding air. At the completion of the test period, the canister is removed from the structure and placed in a gamma counter to determine the amount of radon gas adsorbed. Based on the observed count and the length of time the canister was left in place, an estimate can be made of the average radon gas concentration in the structure during the measurement period.
Unfortunately, the use of charcoal canisters for radon detection has a number of drawbacks. For example, since moisture adsorption by the charcoal in the canister will affect the amount of radon gas which can be adsorbed, the measured radon concentration will vary with ambient humidity conditions unless a correction factor is applied. A correction factor is also required in order to account for the time elapsed between the completion of the test period and the time at which the gamma count is obtained. Another problem is that the canister responds very slowly to changes in ambient radon gas levels, and must be left in place for an extended period of time (typically 2 to 4 days) in order to obtain a reasonably accurate measurement. At the end of the test period, the canister yields only a single count representing the average radon gas concentration during the test period, whereas actual radon levels often vary drastically during the course of a day and on a seasonal basis. This count represents only the concentration of radon gas, which is harmless in itself, and does not reflect the concentration of radon decay products which constitute the actual health risk. Although the concentration of radon decay products (referred to as the radon working level) can be inferred from the measured radon gas concentration under conditions of equilibrium, such conditions are not usually present. Factors such as varying rates of radon gas infiltration, changing ventilation, and the use of home air filters to remove airborne dust particles can disrupt the equilibrium between radon gas and decay product concentrations.
A particularly serious problem with the use of charcoal canisters is their susceptibility to tampering. In the typical situation in which the radon gas level s being checked as part of a real estate transaction, there is a strong incentive on the part of the seller to take steps to lower the measured radon concentration if possible. This can be done quite easily by opening a door or window to increase the ventilation in the area being tested, or by moving the canister outdoors during part of the test period. In most cases, there is no way to verify that tampering has not occurred when the canister is retrieved for measurement, and hence the measured radon level cannot be regarded as completely reliable.
Several proposals have been made for detecting or preventing tampering with charcoal canisters. These have included relatively simple expedients such as sealing doors and windows, and marking the precise position of the canister in order to verify that it has not been moved during the test period. The use of air movement sensors and canister motion sensors has also been suggested, and the use of tracer gases has been proposed as a means for detecting patterns of air movement and the extent to which outside air has entered the measurement area. Unfortunately, however, these techniques are capable of detecting only a few of the more common types of tampering. Moreover, because the canister provides only a single measurement indicative of the average radon gas concentration during the test period, the occurrence of a momentary condition that may or may not indicate tampering, such as movement of the canister, cannot be correlated with instantaneous radon level measurement to determine whether tampering has actually occurred. Thus, the measurement data must be regarded as invalid whenever a possible tamper-indicating condition has occurred during the test period, even if the condition was a momentary or accidental disturbance having no effect on measured radon levels. This results in the loss of valid measurement data and requires that the test be repeated even though no actual tampering has occurred.